The present invention relates to the technical field of electrochemistry and describes a method and an apparatus for producing fuel cell components, in particular for producing membrane electrode units (“MEUs”) for membrane fuel cells (PEMFC, DMFC) but also for other electrochemical devices such as electrolysers or sensors.
Fuel cells convert a fuel and an oxidizing agent spatially separated from one another, at two electrodes, into power, heat and water. Hydrogen, a hydrogen-rich gas or methanol can serve as the fuel, and oxygen or air as the oxidizing agent. The process of energy conversion in the fuel cell is distinguished by a particularly high efficiency. For this reason, fuel cells in combination with electric motors are acquiring considerable importance as an alternative to conventional internal combustion engines. However, they are also increasingly being used for stationary and portable applications.
The polymer electrolyte membrane fuel cell (“PEM” fuel cell) is distinguished by a compact design, a high power density and a high efficiency. The technology of the fuel cells is described in detail in the literature, cf. for example K. Kordesch and G. Simader, “Fuel Cells and their Applications”, VCH Verlag Chemie, Weinheim (Germany) 1996.
A PEM fuel cell stack consists of a stacked arrangement (“stack”) of individual PEM fuel cells, which in turn consist of membrane electrode units (“MEU”s), between which so-called bipolar plates for gas supply and power conduction are arranged. In order to achieve a certain cell voltage, a large number of individual membrane electrode units are stacked one behind the other.
A membrane electrode unit, as described in the present application, has, as a rule, five layers and consists preferably of an ion-conducting membrane which is connected on both sides in each case to an electrode (“5-layered MEU”). Each electrode in turn comprises a gas diffusion substrate, also known as a gas diffusion layer (“GDLs”), which is provided with a catalyst layer.
The catalyst layer on the anode is formed for the oxidation of hydrogen, so the corresponding electrode is referred to as the “anode electrode”, or the “anode” for short.
The catalyst layer on the cathode is formed for the reduction of oxygen. The corresponding electrode is therefore referred to as the “cathode electrode”, or the “cathode” for short.
The gas diffusion substrates (GDLs) are generally based on substrates which permit good access of the reaction gases to the electrodes and good conduction of the cell current. They may consist of porous, electrically conductive materials, such as carbon fibre paper, carbon fibre nonwovens, woven carbon fibre fabrics, metal meshes, metallized fibre fabrics and the like.
For gas-tight sealing of the MEUs on installation in fuel cell stacks, the MEUs may furthermore contain sealing materials, reinforcing materials or optionally protective films in the edge region. In this way, more highly integrated MEU products can also be produced (for example “7-layered MEUs”).
Bipolar plates (also referred to as “separator plates”), which, as a rule, are produced from conductive graphite and have channels for the gas supply and gas removal, are mounted between the MEUs.
Anode and cathode electrodes contain electrocatalysts which catalytically oxygen). As a rule, noble metal-containing catalysts which contain finely dispersed noble metals, such as, for example, platinum, palladium, ruthenium, gold or combinations thereof, are used for this purpose. Carbon black-supported catalysts of the type Pt/C or PtRu/C, which comprise finely dispersed platinum or platinum/ruthenium on a conductive carbon black surface, are preferred. Typical noble metal loads of the catalyst-coated electrodes are from 0.1 to 0.5 mg Pt/cm2 for the anode side and from 0.2 to 1 mg Pt/cm2 for the cathode side. On the anode side, special PtRu-containing catalysts are used for operation with reformate gas.
The ion-conducting membrane preferably consists of proton-conducting polymer materials. A tetrafluoroethylene/fluorovinyl ether copolymer having acid functions, in particular sulphonic groups, is used with particular preference. Such a material is sold, for example, under the trade name Nafion® by E.I. DuPont. However, it is also possible to use other, in particular fluorine-free, ionomer materials, such as sulphonated polyether ketones, sulphonated polyaryl ketones, doped polybenzimidazoles and/or inorganic ionomers.
Various methods for producing components for fuel cells are described in the literature:
EP 1 365 464 A2 discloses a continuous process for producing gas diffusion layers for PEM fuel cells. A laminating method is not mentioned.
EP 1 037 295 B1 describes a method for applying electrode layers to an ionomer membrane in ribbon form by means of a screen printing process.
EP 868 760 B1 discloses a continuous method for producing membrane-electrode composites. In this case, the ion-conducting membrane is laminated and bonded with the contacting material in ribbon form in a roller arrangement.
WO 03/084748 A2 discloses a method and an apparatus for producing membrane electrode units. The MEUs are in this case produced using an ionomer membrane in ribbon form by lamination on both sides with electrodes (i.e. gas diffusion substrates) or catalyst-coated substrates (so-called “decals”). The electrodes or substrates, previously cut to size in a punching device, are transported to the laminating location with the aid of vacuum belts and are laminated there with the polymer electrolyte membrane. This method has the following disadvantages:    a) The vacuum belts used lead to a high degree of complexity of the apparatus, which results in higher costs, complicated measurement and control technology and increased servicing work.    b) The feeding by means of vacuum belts implies transfer locations to the rollers. As a result, the size of the electrodes is limited in the downward direction for geometrical reasons; it is not possible to produce MEUs to any small size that may be desired.    c) The use of vacuum belts limits the heat influencing zone for the electrodes or substrates to the region of the roller nip. This narrow heating zone has the effect that there is insufficient heat transmission during the laminating process, in particular if relatively high production rates have to be realized. The system capacity of such an apparatus is therefore limited.